The present invention relates to chain saws and in particular to the tooth shape of chain saw nose sprockets.
Since the 1950's, saw chains have comprised drive links having a drive tang extending into a groove in a guide bar, and side links on each side of the drive links. Some of the side links are provided with cutter edges and depth gauge projections. The drive tang is approximately triangular, with a cutout at the bottom of its leading edge to catch and convey lubricating oil. Early examples are shown in U.S. Pat. No. 3,180,378, where there is provided a distance between adjacent drive links (still typical for most chain), and in U.S. Pat. No. 3,261,385 where the spacing is much longer.
The chain is powered by a drive sprocket connected to a motor. Since the drive sprocket has a number of teeth which is much less than the number of drive links of the chain, the former being typically 10% of the latter, the drive sprocket teeth will experience much wear. As a drive sprocket wears, its pitch will be reduced. The pitch for a sprocket is defined as the distance between drive links along a chain of a size where every drive link will be fully supported on its leading and trailing edges, and the pitch circle is a circle through the rivet centers of such a chain. When a chain wears, its pitch will increase.
It was early shown in U.S. Pat. No. 2,351,740 that the expected wear of a drive sprocket makes it suitable to start with a sprocket with larger pitch than the chain. That means that most of the tangential force will be carried by the drive link which is about to leave the sprocket. The rest of the chain will be tensioned and its drive links will sequentially extend down into the gullets between the sprocket teeth before they have to carry any tangential force. That is, each drive link will become disposed in a gullet ahead of its respective sprocket tooth before being contacted by that sprocket tooth. This reduces the wear and vibration.
When finally the chain wear causes the chain pitch to be increased so much that it exceeds the sprocket pitch (which has decreased), each of the drive links, at the instant that its tang enters the sprocket gullets, will carry the whole tangential load for a short time until the next drive link enters; the rest of the chain in contact with the sprocket will be without tension and will be prone to vibration. At this point, it is advisable to install a new chain or sprocket.
At the front (nose) of the guide bar is mounted a freely rotatable nose sprocket around which the chain travels, so as to rotate the nose sprocket.
Nose sprockets of the type described in U.S. Pat. No. 3,124,177, were intended to convey the chain around the guide bar nose without any tangential force, and were made with the same pitch as the chains. The nose sprocket teeth were originally made so short that they did not penetrate into a space formed between the side links of the chain, since the undersides of the side links were sometimes beaten and deformed by the drive sprocket teeth to such extent, that the lateral distance between sidelinks was much reduced.
Other types of drive sprockets later reduced this problem, so longer nose sprocket teeth were introduced, but were made rather sharp and with lateral chamfers to ensure that they would enter between the sidel inks. These longer teeth made the chain run more stable around the nose, which was an advantage when making piercing or plunging cuts into the tree trunks. However, longer teeth created problems when the chain pitch was short, as in the case of a new unused chain, and short teeth were again proposed in U.S. Pat. No. 4,970,789.
Having nose sprockets with a pitch equal to or shorter than the chain's nominal pitch has lately been found to be a major cause of sprocket failure in guide bars used for mechanized timber felling or processing. The machines used for mechanized felling normally operate at very high chain speeds and with high chain tension, resulting in high stress in the sprocket, much aggravated by chain wear.
There are several reasons why it is increasingly desirable to accept and adapt to pitch variations of the chain. One is that freshly produced chain normally has minor shape variations such as jarred edges in the rivet holes, which will become smooth after some running time and result in some initial lengthening of the chain. Manufacturers tried to deal with this by a pre-running-in of chain loops for a break-in period before delivery, as a last step in manufacture, but this was a costly and time-consuming procedure.
Some lengthening of the chain during actual use is inevitable, however, due to the wear of rivets and rivet holes. To make it possible to use a chain for as long a time as its cutting edges will last, the nose sprocket will have to accept this variation.
A chain with shorter pitch than the nose sprocket cannot be arranged as a circular (i.e., constant-radius) arc at constant radius around the sprocket, but has to be arranged as a spiral (non constant-radius) arc with one link at a shortest radial distance from the sprocket center, and the others at gradually increasing distances. The link with shortest radial distance will be the only fully supported link. During actual running, this is usually the link that has traveled the whole arc and is about to leave the sprocket, as is shown in U.S. Pat. No. 3,683,980. The trailing links will be supported at their leading edge only. Throughout the arc, the chain will be under relatively constant tension, and the drive link tangs will enter the gullets without impact and gradually slide smoothly down.
A chain with longer pitch than the nose sprocket cannot be arranged with all links supported, either as a circular arc or as a spiral arc. The link about to leave the sprocket will be supported at its leading edge; the link about to enter the sprocket will be supported at its trailing edge; and the intermediate links will be without support. The entering of the link within the sprocket will occur with an impact as the whole chain tension will have to be transferred to the sprocket. The exiting of the link from the sprocket will occur as a sliding motion with high contact pressure during a short time. If continued, this will soon lead to fatigue failure of sprocket or chain. Furthermore, the untensioned intermediate links will be very unstable and may easily cause kick-back if they touch the wood as in plunge cutting.
One early attempt to ease the entry of the drive link tangs into the gullets was suggested in U.S. Pat. No. 3,263,715, where the whole contact surface of the tooth was a convex circular arc. This did not make it possible to make the teeth long enough to enter between the sidelinks, however, and the supported position of the links was undetermined since the link tangs had straight edges.
When running an equal or slightly shorter pitch chain over a nose sprocket, the crucial instance is when a link is about to enter the sprocket. When a link is about to settle on the sprocket, the contact point at the leading edge of the link will move along a curve consisting of successive circular arcs centered at successive rivets of links which have already settled, until it touches the tooth. If the tooth contact surface continues straight above the pitch circle, it will ultimately cut this curve which may lead to an impact on the leading edge of the settling link and a temporary increase of the tension force among the settled links. The risk of this is more acute for shorter pitch chain, and in the prior art this set the limit both for how great of a pitch difference could be accepted and how long the teeth could be made.